Along with the greatly increased use of computers for offices and for manufacturing facilities, there has developed a need for connecting peripheral equipment to mainframe computers. A system which includes a cable for accomplishing the interconnection has long been sought. A number of factors must be considered in order to arrive a cable design which is readily marketable for such a use.
Cable connectorability is very important and is more readily accomplished with twisted conductor pairs than with any other medium. Cable jackets should exhibit low friction to enhance the pulling of cables into ducts or over supports. The cables should be strong, flexible and crush-resistant, and they should be conveniently packaged and not unduly weighty. Flame retardance also is important.
To satisfy present, as well as future needs, these sought after cables should provide suitable high frequency data transmission. This requires a tractable loss for the distance to be covered, and crosstalk and electromagnetic interference (EMI) performance that will permit substantially error-free transmission. Also, the cables must not contaminate the environment with spurious radiations.
The sought-after system including the data cable should be low in cost. The system including the cable must provide for suitable transmission, yet be capable of being economically installed, be efficient in terms of space required, and be durable.
Installation cost is an important factor. Generally, for cables in buildings, which are used for such interconnection, installation costs outweigh the material costs. Installation costs are affected by the cross-sectional area of the cable. Building cables should have a relatively small cross-section inasmuch as small cables not only enhance installation but are easier to conceal, require less space in ducts and wiring closets and reduce the size of the associated hardware.
Also important to cost is whether or not the system is arranged to provide transmission in what is called a balanced mode. In balanced mode transmission, voltages and currents on the conductors of a pair are equal in amplitude but opposite in polarity. This requires the use of additional components, such as transformers, for example, at end points of the cable between the cable and logic devices thereby increasing the cost of the system. Generally, computer equipment manufacturers have preferred the use of systems characterized by an unbalanced mode inasmuch as most of the industry is not amenable to investing in additional components for each line. In an unbalanced mode transmission system, voltages and currents on the conductors of a pair are not characterized by equality of amplitude and opposition of polarity. Computer equipment manufacturers have adopted unbalanced mode transmission and do not intend to retrofit their equipment in order to simplify the interconnection of peripherals. At the same time, however, peripheral connection arrangements must meet predetermined attenuation and crosstalk requirements.
Of importance to the design of local area network copper conductor cables are the distances over which signals must be transmitted. Often, this need is one for interconnection over a distance of about one hundred and fifty feet or less. This need at transmission rates of 20 kilobits per second or less has been satisfied in the prior art with single jacket cables which may comprise twisted pairs of conductors which are connected directly between a computer, for example, and receiving means such as the peripheral equipment. Additional components at the ends of each pair to convert the transmission to the balanced mode have not been used.
Shielding often is added to each twisted pair to confine its electric and magnetic fields. In this way, susceptibility to electromagnetic interference is reduced. However, as the electric and magnetic fields are confined, resistance, capacitance and inductance all change, each in such a way as to increase transmission loss. For example, the attenuation of one widely used, individually shielded local area network data pair is 50% greater than it would be if it were one of many pairs having a common shield in a typical exchange cable.
On the other hand, a cable shield surrounding all conductor pairs in a cable may be advantageous. Consider that the pairs may be inside a cabinet and may be exposed to high speed digital signals. Stray radiation will be picked up in the longitudinal mode of the twisted pairs. If the pairs are then routed outside the cabinet, they may radiate excessively. If there is a cable shield enclosing the plurality of pairs, the shield may be grounded at the cabinet wall so that the shield will not itself carry stray signals to the outside environment. Thus, a shield disposed about all the pairs in a cable can be effective in preventing electromagnetic interference and yet not increase appreciably the attenuation of each pair.
An escalating need has developed for longer length cables to permit peripheral equipment to be spaced farther from its associated mainframe computer. Studies have shown that almost all station-to-serving closet runs are less than about 200 feet. Given this maximum distance, other parameters of local area network copper conductor cables can be optimized. In the range of distances up to about 200 feet, this need has been filled with coaxial cable comprising the well-known center solid and outer tubular conductor separated by a dielectric material.
The use of coaxial cables, which inherently provide unbalanced transmission, presents several problems. To connectorize coaxial cable is very expensive. Further, for the data transmission arrangements which are contemplated, eighteen conductor pairs or the equivalent thereof are required for each cable. To have eighteen coaxial cables and the connectors associated therewith requires the dedication of an undue amount of building space which is at a premium cost. Of course, the use of coaxial cables does not require components such as transformers at each end to provide balanced mode transmission, but the size and connectorization of coaxial cables outweigh this advantage.
Seemingly, the solution of the prior art to the problem of providing a data transmission system between a mainframe computer and peripheral equipment are not totally satisfying in today's world. What is needed and what is not provided by the prior art is a data transmission system which is compatible with unbalanced mode transmission computer equipment but which does not involve the need for additional components at the terminus of each line. Further, any arrangement proposed as a solution to the problem should be one which does not occupy an undue amount of space and one which facilitates a simplistic connection arrangement. There is a need to provide cables that can transmit data streams of many megabits, error free, from stations to closets or between computer cabinets separated by comparable distances to main rooms, be readily installed, fit easily into building architectures, and be safe and durable.